Perimeter, Area, Circumference & Volume
Perimeter is the distance around a figure (add the sides); area measures the surface inside (rectangle: length x width; triangle: half base x height; circle: pi r squared); circumference is a circle's perimeter (2 pi r); volume fills a solid (box: length x width x height).
Area answers come in square units and volume in cubic units — a quick units check catches many wrong answers. For composite figures, split the shape into rectangles and triangles, find each piece, and add.